This invention concerns a process and apparatus for preparing vegetable oilseed meal for subsequent solvent extraction of the contained vegetable oil. In processing vegetable oilseed on an industrial scale to produce vegetable oil and de-oiled meal, the usual final step is to extract oil from a prepared meal using a solvent such as hexane.
Some seeds, notably soya beans and palm kernels can be directly extracted after suitable pre-treatment by breaking, heating and rolling. However, many seeds are pressed prior to solvent extraction to remove the easily-won oil so that a "cake" of lower oil content, which is more readily extractable, is presented to the solvent extraction system. Seeds processed in this way include, for example, rapeseed, sunflower, groundnuts, cottonseed, and copra. The pressed meal, known as presscake is fed to the extractor with a residual oil content in the range 16% to 20%. The presscake is prepared by passing the vegetable oilseed through a machine in which it is advanced through a perforated barrel by a screw assembly so that its temperature is elevated slightly, and oil squeezed from the meal passes through the perforated barrel for collection.
The subsequent solvent extraction process is designed to produce a minimum oil content in the meal which is generally in the range 0.5 to 1.5%, and a minimum retention of solvent which is generally in the range 200-1000 ppm. The oil-in-meal percentage and the solvent retention are measures of the "extractability" of the material fed to the extraction process.
The present invention is based upon an appreciation that the extractability of many oilseeds can be improved by passing the pressed meal through an expander/extruder prior to solvent extraction.
An expander/extruder is a machine similar to a screwpress, but without a perforated barrel. The moisture level of the meal is increased with respect to that at which it normally leaves the barrel of a screw press, and the material is compressed and heated by the action of a worm assembly before it is extruded through one or more dies. The moisture in the material remains liquid within the barrel due to the high pressure generated therein, but as it emerges through the die, the moisture immediately flashes off, provided the temperature of the material is above 100.degree. C. The sudden expansion and evacuation of moisture causes the material to become porous, which significantly enhances the subsequent rate of extraction by solvent, and the ease with which the solvent can be removed from the material after the extraction process.
An object of the present invention is to provide a process and system for preparing vegetable oilseed meal for subsequent solvent extraction, wherein the extractability of the material is increased with minimal energy requirement.
According to the present invention there is provided a process for preparing vegetable oilseed meal for subsequent solvent extraction of the contained vegetable oil, comprising the steps of introducing the meal into the inlet end of a machine having a screwpress section and an expander section, and in which it is advanced and worked by a worm and worm shaft rotating within a perforated barrel wall in the screwpress section and in a non-perforate barrel wall in the expander section, the meal passing from the screw press section directly into the expander section, increasing the moisture content of the meal passing into the expander section, and extruding the meal from the expander section through a restricted orifice.
Further according to the invention there is provided a system for preparing vegetable oilseed meal for subsequent solvent extraction of the contained vegetable oil, comprising a screwpress section and an expander section, a common worm shaft rotating within a barrel throughout said two sections and having a worm assembly thereon to advance and work meal fed to an inlet end of the screwpress section, a restricted orifice at an outlet end of the expander section, said barrel wall being perforated in the screwpress section for drainage of oil expressed from the meal, and being non-perforate in the expander section, there being means for increasing the moisture content of the meal passing from the screwpress section to the expander section.